Ribosomes of ovulated mouse ova analysed on sucrose gradients are present primarily as typical monomers and subunits, while 20 to 25% are in polysomes. Those not in polysomes appear to be unable to form a translation complex with poly U. The content of poly A-containing RNA has been measured on poly U sepharose columns, and is found to be 9 to 10% of total RNA of ovulated mouse ova. Preliminary experiments on the loss of maternal RNA during embryogenesis show that about 40% disappears by the 2-cell stage, and another 30% by the blastocyst stage.